Forwards/backwards processing is a post-processing method used to improve positioning accuracy in GNSS/INS navigation systems. This method leverages the strengths of both technologies by using all collected data in two directions: forward and backward. Forward processing analyzes data in chronological order, just as it was recorded. Backward processing does the reverse, reading and analyzing data from the last recorded point back to the beginning. This dual-pass technique provides two trajectory estimates, which can then be merged to enhance accuracy.
GNSS offers high long-term accuracy but fails in areas with signal obstruction like tunnels or dense cities. Inertial systems, on the other hand, deliver reliable short-term data but suffer from drift over time. When GNSS signals drop out, such as in a tunnel, inertial data maintains the trajectory but introduces errors. These errors accumulate differently when processed backward. By comparing both forward and backward inertial paths, the system identifies and reduces drift effects.
Imagine a vehicle entering a tunnel where GNSS signal is lost. The forward-processed path shows accumulated drift by the time the vehicle exits. When processed backward from the tunnel exit, drift appears at the entrance instead. Combining both directions balances errors and improves the final path estimate. This approach helps capture key movements, like lane changes or obstacle avoidance, even during GNSS outages.
This technique also enhances attitude estimation, improving pitch, roll, and yaw accuracy. It works in three dimensions, not just laterally. While not essential for very short outages, it always improves data quality. Merging forward and backward results produces a robust and accurate final trajectory.
Forwards/backwards processing is ideal for applications where high accuracy is critical. It turns a GNSS/INS system into a more powerful solution, especially in challenging environments where GNSS alone would fail.
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